It is known that the continuous, three-phase reactors comprising a heterogeneous catalyst and used industrially are stationary bed reactors with two-phase flow or fluidized bed reactors comprising three phases.
The most widespread type of catalytic reactor is that where the fluid phases pass through a stationary bed composed of a stack of catalyst particles. Three configurations can be envisaged, according to the respective directions of flow of the gas and liquid phases:                downward cocurrent,        upward cocurrent,        countercurrent.        
Stationary bed catalytic reactors with downward cocurrent flow are the most widespread on the industrial scale. They ensure that the catalytic bed is kept immobile and, for this reason, they are preferred to the upward cocurrent systems, for which the movement of the catalyst particles under the effect of the flow leads to unacceptable attrition. On the other hand, stationary bed and downward cocurrent flow reactors exhibit failings with regard to distribution of the liquid which are solved in the case of the upward flow systems.
Three-phase reactors with countercurrent flow are difficult to make use of industrially because of the phenomenon of flooding, which severely limits the flow rates of each of the phases which have to pass through the catalytic bed. In this case, the solution consists in using larger catalyst particles than those accepted in stationary beds, which unfortunately has the consequence of reducing the effectiveness of the said catalyst.
Turbulent or fluidized bed three-phase reactors make it possible to improve fluid-catalytic solid transfers by using a material with a very fine particle size. In this case, the liquid and gas phases pass through the reactor from the bottom upwards and combine to keep the solid particles, with a size generally of between 0.1 and 5 mm, in suspension. In comparison with a stationary bed, the fluidized bed exhibits two advantages: better control of the temperature and the possibility of extracting the catalyst during operation in order to replace it. On the other hand, a major difficulty appears in the ratio of the acceptable gas and liquid flow rates. This is because fluidization is easily achieved in the case where the velocity of the liquid is relatively high whereas the velocity of the gas remains low. In the reverse case, a system of pulsed type, with alternation of concentrated and dilute regions, is established. Partial recycling of the liquid phase, carried out by means of an immersed or external pump, makes it possible to solve this problem but at the expense of the efficiency of the reactor. Furthermore, a sufficient height has to be provided in the top part of the reactor, above the upper level of the catalyst bed, in order to avoid entrainment of the particles.
The two types of three-phase reactors which are stated above have already been employed, as described above, in the field of the treatment of water by oxidation in the presence of heterogeneous catalyst.
For example, WO 90/14312 and FR-A-2 291 687 relate to processes for ozonization with solid catalysts positioned as column packing and fed as an upward cocurrent with the liquid to be treated and the ozone-comprising gas.
In FR-A-2 269 167, the granular ozonization catalyst is inserted between stationary layers of inert material to fill a reactor comprising four chambers in series operating as an upward cocurrent system.
WO 96/21623 and WO 97/14657 provide for catalytic ozonization of wastewater to be carried out in a stationary bed column with prior dissolution of the ozone in the liquid phase in an upstream contactor, that is to say in a two-phase reactor.
U.S. Pat. No. 4,007,118 discloses a plant or effluent treatment by ozonization in a bubble contactor operating countercurrentwise with flows of the liquid downwards and of the gas upwards through a catalyst present in immersed bags or deposited on a stationary support or dispersed in the reaction medium and recycled after separation from the treated water.
EP-A-0 625 482 employs a treatment of polluted water by oxidation in the presence of ozone in a treatment column in an upward cocurrent system with fluidized or stationary catalytic bed.
The design of stationary or fluidized catalytic bed three-phase reactors for the oxidative treatment of polluted water must take into account not only the constraints related to the properties of the catalyst (activity, selectivity and regenerability) but also variations in flow rate, in polluting load or in concentrations of suspended matter.
These three-phase reactors exhibit a number of disadvantages which will be mentioned below.
Stationary Bed Reactors:
Although the stationary bed and two-phase flow reactor exhibits the advantages of being simple to use, of easy separation of the phases and of “plug” flow of the phases, the problems of extrapolation prove to be complex. In particular, the homogeneous distribution of the liquid phase has to be rapidly achieved in order to avoid having a catalytic bed dead height with rapid deactivation of a portion of the catalyst by accumulation solid particles or of poisons.
The maintenance of good distribution of the phases, critical in downward cocurrent reactors, involves operating conditions characterized by strong interaction of the gas and liquid phases and therefore the use of a ratio of the gas/liquid flow rates of greater than 10, very high for an application in water treatment.
It is markedly preferable for the reactor to operate with an upward cocurrent of gas and of liquid; however, in this case, phenomena of backmixing of the reaction phase and the imperfect distribution of the gas inevitably appear. This is because the gas is conveyed into the catalytic bed in the form of a train of bubbles with a diameter of approximately 3 mm, the upward rate of which varies between 10 and 20 cm/s. Furthermore, the three-phase reactors with a stationary bed and an upward two-phase flow are characterized by a limited gas-liquid transfer, resulting from the low value of the interfacial area (the gas/liquid contact surface area is reduced in the case of bubbles over a large diameter), which governs the dissolution of the oxidizing gas in the liquid and consequently the rate of oxidation. Finally, and generally, it is known that the major problem encountered in the use of three-phase stationary bed reactors is the significant decrease in activity of the catalyst, resulting from the decline in the intra- and extragranular material transfer phenomena relative to the size of the catalyst particles. In both cases of cocurrent flow, the material transfer has to be optimized by establishing a strong interaction between the gas and liquid phases. This involves the use of a high gas flow rate with respect to the flow rate of liquid to be treated and therefore a large excess of oxidizing gas with respect to the pollutant, consequently resulting in a low oxidation yield.
Another major disadvantage of stationary bed catalytic reactors for application in water treatment lies in the retention of the suspended matter present in the water to be treated. According to the direction of flow of the liquid phase, the suspended matter is retained in the bottom part or in the top part of the catalytic bed and it is then necessary to periodically discharge it by expanding the bed, for example by means of a strong gas flow. This washing treatment necessarily involves halting the operation of the reactor, on the one hand, and furthermore subjects the catalyst to stresses which may be incompatible with its mechanical strength.
Fluidized Bed Reactors:
The implementation of the catalytic oxidation reaction in a fluidized bed reactor makes it possible, by using particles with a smaller particle size than that employed in stationary bed reactors (up to 0.1 mm), to provide better gas-liquid solid transfer. Another advantage of the fluidized bed lies in the possibility of continuously regenerating the catalyst in the event of rapid deactivation. In practice, for the catalytic oxidation applied to water treatment, the two fluid reaction phases, mixed or unmixed, are injected at the base of the bed present in the treatment column. According to this scheme, which is nevertheless conventional, the technology of the fluidized bed results in numerous disadvantages.
The behaviour of the fluidized three-phase reactor corresponds to that of a stirred reactor, which involves increasing the reactional volume (with respect to a three-phase reactor with a stationary catalyst bed) in order to observe the contact time necessary for the catalytic reaction, this increase in volume leading to the appearance of unfavourable backmixing phenomena.
Finally, the suspending of the catalyst particles requires, above the reaction region, a significant “disengagement” or separation region for reducing the inevitable entrainment of the catalyst particles in the treated water. Apart from these considerations, the operation of this type of reactor also requires the precise adjustment of the gas and liquid velocities to maintain the fluidization and to prevent the appearance of intermediate situations harmful to the performance of the treatment. The hydrodynamic equilibrium corresponds to precise gas and liquid flow rates, involving inflexible operation, and only allows small variations which are not easily compatible with continuous operation in the context of the treatment of wastewater of eminently variable composition.